1. Field of the Invention
The invention is in the field of flow rate and fluid measurement systems and is particularly directed to fuel measurement systems utilizing capacitive probes.
2. Description of the Prior Art
Present vehicle fuel measurement techniques are relatively inaccurate and are commonly expressed in terms of fractions of tank capacity thereby providing only a gross fuel estimate. Fuel flow rates are generally determined by relatively expensive positive displacement mechanisms such as mechanical pistons or rotary impellers. Positive displacement devices, as represented, for example, by U.S. Pat. Nos. 4,043,198 and 2,304,822 depend upon float mechanisms and/or valve switch mechanisms. Some types of flow rate measurement systems utilize a reservoir or a "subtracting tank" in conjunction with positive displacement mechanisms as described, for example, in U.S. Pat. Nos. 3,332,283, 3,695,097, 3,714,823 3,937,083 and 4,020,683. These types of flow devices are not capable of measuring remaining or added fuel per se or change in fuel tank levels. Furthermore, the accuracy and ruggedness of these mechanisms are affected by dynamic forces which include vibration, shock, vehicle acceleration, fluid dynamics, etc.
It has been known to utilize capacitive probes for various fuel measurements as illustrated by U.S. Pat. Nos. 2,354,964, 2,656,977, 2,767,360, 2,772,049, 4,090,408 and 3,918,306. Such systems are generally not flexible enough to be utilized to measure both flow rate and consumed or remaining fluid, and these systems largely depend upon complex and expensive analog apparatus which gives only course measurements. Digital processing techniques have been known for utilization with various types of sensors such as velocity sensors and fuel sensors. Among teachings of this sort are U.S. Pat. Nos. 3,908,451, 3,921,444, 3,927,305, 3,983,372, 3,975,952, 4,061,023, 4,112,492 and 4,136,563.
U.S. Pat. Nos. 4,134,301 and 4,073,186 disclose microprocessors utilized in non-capacitive type flow meter systems, and U.S. Pat. No. 4,083,248 shows a capacitive probe and a read only memory for providing a table lookup feature for compensating for probe non-linearities.
The above patents utilizing the subtractive tank technique as well as U.S. Pat. No. 3,423,998 are particularly directed to measurements of recirculating fuels as found for example in diesel engines. In such engine systems, a large percentage of the intake fuel is not burned by the engine but is rather returned to the fuel tank. Fuel measurements are made difficult since the returned fuel is generally hot, foamy and unstable (a condition sometimes referred to as slug flow).